11.49 More on predicting the total amount of PCB. Run a regression to predict PCB using the variables PCB52, PCB118, and PCB138. Note that this is similar to the analysis that you did in Exercise 11.47, with the change that PCB180 is not included as an explanatory variable.

  1. (a) Summarize the results.

  2. (b) In this analysis, the regression coefficient for PCB118 is not statistically significant. Give the estimate of the coefficient and the associated P-value.

  3. (c) Find the estimate of the coefficient for PCB118 and the associated P-value for the model analyzed in Exercise 11.47.

  4. (d) Using the results in parts (b) and (c), write a short paragraph explaining how the inclusion of other variables in a multiple regression can have an effect on the estimate of a particular coefficient and the results of the associated significance test.